This application claims priority to an application entitled, xe2x80x9cMultimode Optical Fiber Having a Structure to Reduce Scattering Lossxe2x80x9d, filed with the Korean Industrial Property Office on Mar. 22, 2000 and there duly assigned Serial No. 2000-14517.
1. Field of the Invention
The present invention generally relates to an optical fiber, and more particularly to a multimode optical fiber having a structure to reduce scattering loss.
2. Description of the Related Art
In general, an optical fiber is classified into a step index fiber and a graded index fiber. The step index optical fiber is generally comprised of a core with the refractive index n1 and a cladding with the refractive index n2. In step index fiber, the refractive indices of the core and the cladding respectively maintain a constant value. However, the refractive index profile of the core changes gradually with distance from the fiber axis in the graded index optical fiber. The varying characteristics of the refractive indices n(r) from the fiber axis along the radial direction are described according to the following equation 1:
n(r)=n1{square root over (1xe2x88x922(r/xcex1)xcex1)}xcex94rxe2x89xa6a,
n(r)=n2{square root over (1xe2x88x922xcex94)}r greater than axe2x80x83xe2x80x83(1)
Herein, n1 represents the refractive index of the center of the core, n2 represents the refractive index of the cladding, a represents the radius of the core, xcex1 represents the core shape index, and xcex94 represents the parameter determining the degree of the refractive index variation.
Here, only in the case of n1=n2, the aforementioned xcex94 is approximated according to the following equation 2:                               Δ          =                                    (                                                n                  1                                -                                  n                  2                                            )                                      n              1                                      ,                            (        2        )            
where xcex94 is defined as a relative index difference.
FIG. 1 is a sectional view illustrating the refractive index profile of the graded index fiber according to the prior art. FIG. 1 illustrates the graded index fiber 10, including the core 11 and the cladding 12, and the ideal refractive index profile 13 as well as the real refractive index profile 14 of the graded index fiber 10. In ideal refractive index profile 13, the region of the core 11 can be defined by the equation 1. In actual practice, however, the center of the core 11 with a recess represented by the refractive profile 14, as shown in FIG. 1, is realized. Here, the recessed shape is caused during the fabrication of an optical fiber preform.
Typically, a chemical vapor deposition method is used in fabricating an optical fiber preform. In this method, after the cladding of the optical fiber preform is prepared, the core is formed by injecting and sintering gasses into the cladding. Here, the core is not filled up to the center, thus a hole having a specified diameter is formed within the glass tube. Thereafter, the optical fiber preform is completed finally through a collapsing. As a consequence of this collapsing process, an optical fiber drawn from the optical fiber preform exhibits a real refractive index profile 14 as shown in FIG. 1.
Normally, the refractive index variation degree of the graded index fiber 10 can be adjusted by the type and the amount of dopant added to the fiber. Common dopant material used include titanium, germanium, boron, and etc. However, the dopant over the graded index fiber 10 causes scattering loss of the optical signal within the graded index optical fiber 10. Hence, as the composition rate of the dopant is increased, the scattering loss of the optical signal also increases.
It is, therefore, an object of the present invention is to provide an improved multimode optical fiber for eliminating the scattering properties.
In the preferred embodiment, a multimode optical fiber having a structure to reduce scattering loss includes a core divided into a central region with an optimal refractive index and a peripheral region whose refractive index is lower than the central region of the core, and a cladding enclosing the core, wherein the cladding having an refractive index value lower than the lowest refractive index value of the core. Preferably, the refractive index of the central region and the peripheral region range between 0.5 and 2.5.